1. Field of the Invention
This invention relates to a vapor phase growth apparatus that a source gas is flown onto a semiconductor wafer held by a susceptor so as to grow a semiconductor crystal thin film (epitaxial layer) on the surface of the semiconductor wafer, and particularly to a planetarium type vapor phase growth apparatus that the susceptor rotates on its axis and revolves around an axis (in a certain orbit).
2. Description of the Related Art
A vapor phase growth method is one of methods for growing a semiconductor crystal. In the vapor phase growth method, a source gas is flown onto the surface of a heated semiconductor wafer, and thereby a semiconductor crystal thin film is grown on the surface of the semiconductor wafer. This method is characterized by that an ultra-thin film as thin as several nanometers can be grown since it uses gas as a raw material. Further, the method has a good mass productivity since it uses high-purity organic metal/hydride/carrier gas and it needs no ultrahigh vacuum required of the molecular beam epitaxy.
One of vapor phase growth apparatuses used for the vapor phase growth method is a planetarium type vapor phase growth apparatus that the susceptor rotates on its axis and revolves around an axis (in a certain orbit). The planetarium type vapor phase growth apparatus is characterized by that it has a good uniformity in the thickness of grown film.
FIG. 1 is a schematic cross sectional view showing a conventional planetarium type vapor phase growth apparatus 101.
The vapor phase growth apparatus 101 is structured such that a source gas 103 is introduced into the apparatus from the bottom through a source gas inlet 102 that opens downward. The source gas 103 is thermally decomposed in the apparatus. An exhaust gas 104 thermally decomposed is, in the horizontal direction, discharged radially from the center of the apparatus through a plurality of (e.g., six) gas exhausts 105 which are provided on the sidewall of the apparatus.
In the apparatus, there are provided rotation susceptors 107 to hold a semiconductor wafer 106, a revolution susceptor 108 (for moving the rotation susceptors 107 in orbit motion) and a heater 109 to heat the semiconductor wafer 106. The revolution susceptor 108 is connected to a shaft 111 of a motor 110 at the center thereof such that it is rotated by the driving force of the motor 110. The semiconductor wafer 106, which has a front surface and a back surface, is held by the rotation susceptor 107 while keeping the front surface downward. The rotation susceptor 107 is rotatably mounted on the revolution susceptor 108 through bearings 112.
FIG. 2 is an enlarged cross sectional view showing circled part C in FIG. 1.
The rotation susceptor 107 and the revolution susceptor 108 each are provided with a groove for placing the bearing 112 therein on the opposed surfaces. The bearing 112 is placed between the grooves. The rotation susceptor 107, which is rotatably mounted on the revolution susceptor 108 through the bearing 112, is integrally provided with a pinion gear 113 that protrudes sideward from the position of the bearing 112 and the groove for placing the bearing 112 therein. On the other hand, corresponding to the pinion gear 113, an internal gear 114 is provided on the inner wall of the vapor phase growth apparatus 101 such that the pinion gear 113 meshes with the internal gear 114. 115 is a gear mesh portion where the pinion gear 113 meshes with the internal gear 114. FIG. 4 shows the enlarged gear mesh portion 115 in top view.
FIG. 3 is a top view showing the positional relationship among the internal gear 114 on the inner wall of the vapor phase growth apparatus 101, the rotation susceptor 107 and the revolution susceptor 108. Meanwhile, since FIG. 1 is illustrated schematically, the dimensions of parts in FIG. 1 are not always identical with those in FIG. 3.
As shown in FIG. 3, the vapor phase growth apparatus 101 is provided with the twelve rotation susceptors 107 which are annually disposed on the periphery of revolution susceptor 108 which is formed like a large disk. The rotation susceptors 107 each are provided with six claws 116 by which the semiconductor 106 is held.
In operation, when the revolution susceptor 108 is rotated by the motor 110, the rotation susceptors 107 are also rotated around the center axis of the revolution susceptor 108. Simultaneously, since each of the rotation susceptors 107 also meshes with the internal gear 114, it rotates on its center axis. Thus, the semiconductor wafer 106 being held by the rotation susceptor 107 is allowed to rotate on its center axis while rotating around the center axis of the revolution susceptor 108. In this state, the source gas 103 is introduced through the source gas inlet 102 as shown in FIG. 1, and then it is thermally decomposed on the surface of the semiconductor wafer 106 being heated by the heater 109 such that a semiconductor crystal thin film is grown on the surface of the semiconductor wafer 106. Thus, since the semiconductor wafer 106 rotates on its center axis while rotating around the center axis of the revolution susceptor 108, the high uniformity semiconductor crystal thin film can be grown on the surface of the semiconductor wafer 106.
Japanese patent application laid-open No. 10-219447 discloses an example of the conventional planetarium type vapor phase growth apparatus, though this apparatus is not directly relevant to this invention.
The conventional planetarium type vapor phase growth apparatus as shown in FIGS. 1 to 4 has a problem as mentioned below.
The conventional vapor phase growth apparatus 101 is structured such that the pinion gear 113 integrated with the rotation susceptor 107 to hold the semiconductor wafer 106 is located laterally from the position of the bearing 112 and the groove for placing the bearing 112 therein. Therefore, the diameter of the rotation susceptor 107 is unnecessarily larger than that of the semiconductor wafer 106. Referring to FIG. 3, provided that the semiconductor wafer 106 with a diameter of 76 mm is held by the rotation susceptor 107, the number of the rotation susceptors 107 allocable on the periphery of the revolution susceptor 108 is limited to twelve in view of the positional relationship with the internal gear 114. However, if the number of the rotation susceptors 107 allocable on the revolution susceptor 108 increases, the number of semiconductor wafers, with a semiconductor crystal thin film formed thereon, fabricable by the one vapor phase growth apparatus 101 in one manufacture process increases. Thus, it is desired to increase the number of the rotation susceptors allocable on the periphery of the revolution susceptor to improve the productivity of the vapor phase growth apparatus.